Plasma arc torches are commonly used for cutting, welding, surface treating, melting, or annealing a metal workpiece. Such working of the workpiece is facilitated by a plasma arc that extends from the plasma arc torch to the workpiece. In one type of plasma arc torches, a shielding gas is used to surround and control the plasma arc. In contrast, in another type of plasma arc torches, water is used to surround and control the plasma arc. The gas or water that is used to surround and control the plasma arc generated by a plasma arc torch is typically also used to cool a nozzle assembly of the plasma arc torch. Water has a higher coefficient of heat transfer than gas; therefore, plasma arc torches that utilize water to cool their nozzle assemblies can typically operate at higher currents and therefore provide higher quality cuts than torches that utilize gas for cooling their nozzle assemblies. Plasma arc torches that utilize water as discussed above typically include water-injection nozzle assemblies. Examples of plasma arc torches with water-injection nozzle assemblies are disclosed in U.S. Pat. No. 5,747,767; 5,124,525 and 5,023,425, which are assigned to the assignee of the present invention.
A typical plasma arc torch that includes a water-injection nozzle assembly may further include a torch body defining a longitudinal discharge axis and an electrode secured to the torch body and having a discharge end. The water-injection nozzle assembly is mounted adjacent to the discharge end of the electrode. A typical water-injection nozzle assembly may include a metal inner nozzle member and a metal outer nozzle member that is radially outward from the inner nozzle member. The inner nozzle member defines a gas-constricting bore and the outer nozzle member defines a water-constricting bore. The nozzle members are fit together so that the bores are coaxially aligned with the longitudinal discharge axis defined by the torch body, and a water passageway is defined between the interior surface of the outer nozzle member and the exterior surface of the inner nozzle member.
A typical plasma arc torch includes an electrical source for generating an electrical arc that extends from the discharge end of the electrode. The water-injection nozzle assembly is separated from the electrode by a gas passage proximate to the discharge end of the electrode, and a vortical flow of a gas is provided through the gas passage. The electrical arc ionizes the gas to create the plasma arc, which extends along the longitudinal discharge axis and through the bores of the nozzle members to the workpiece. A water flow source supplies a vortical flow of water to the water passageway defined between the inner and outer nozzle members. The vortical flow of the water exits the water-constricting bore and constricts the plasma arc.
Concentricity of the inner and outer nozzle members is very important to proper operation of a plasma arc torch. U.S. Pat. Nos. 5,747,767 and 5,124,525 disclose inner and outer nozzle members that are press-fit together, by way of metal-to-metal contact, to center and maintain concentricity between the bores of the inner and outer nozzle members.
Avoiding "double arcing" is also important to proper operation of a plasma arc torch. Double arcing may occur when the workpiece, or molten splatter from the workpiece, accidentally contacts the metal outer nozzle member. When this happens, a second plasma arc, in addition to the main plasma arc, extends from the electrode through the inner nozzle member and the outer nozzle member, and ultimately to the workpiece. Insulating the outer nozzle member can reduce double arcing. For example, U.S. Pat. No. 5,124,525 discloses an outer nozzle member having a radially exterior surface and an outer insulating element secured onto the exterior surface of the outer nozzle member. These types of insulating elements are often formed of a ceramic material. Such ceramic insulating elements are somewhat brittle and are therefore subject to being broken when they come into contact with the workpiece or molten splatter from the workpiece.
Accordingly, there is a need for a water-injection nozzle assembly with an insulated front end that is less prone to breakage.